For facilitating understanding, the following concepts are first explained.
Multicast: a communication manner in contrast with unicast and broadcast. A multicast system includes a multicast source, one or multiple receivers and a multicast group. The source host sends data only once, and the destination address of the data is the address of the multicast group. Each receiver in the multicast group receives a copy of the data, and only the receivers in the multicast group can receive the data.
IGMP (Internet Group Management Protocol): a protocol for managing IP multicast members in the TCP/IP (Transmission Control Protocol/Internet Protocol) protocol family, which is for establishing and maintaining relations of multicast group members between an IP host and adjacent multicast routers.
IGSP (Internet Group Management Protocol Snooping): a multicast restriction scheme running in layer-2 devices, which is for managing and controlling multicast groups. A layer-2 device which runs IGMP snooping analyzes a received IGMP packet, establishes a mapping relation between a port and a multicast MAC (Media Access Control) address, and forwards multicast data according to the mapping relation.
PIM: a protocol which provides IP multicast routing by utilizing a unicast routing table established using static routing or any unicast routing protocol such as RIP, OSPF, IS-IS, BGP and so on, and is for maintaining propagation of multicast data between layer-three devices. PIM protocol has four modes, i.e., PIM-DM (Dense Mode), PIM-SM (Spare Mode), PIM-SSM (Source-Specific Multicast) and bi-directional PIM.
IP multicast techniques effectively solve problems existing in one-to-many transmissions, implement efficient one-to-many data transmission in an IP network, and can save a large amount of network bandwidth and reduce network loads. Some new value-added services, including Internet information services, such as online live broadcast, IPTV, remote education, Telemedicine, internet radio, real-time video conference, and so on, are able to gain greater development by taking advantage of the multicast functions of the network.